Recently, as a backlight of a projection-type image display such as a liquid crystal projector, an overhead projector and the like, a projection-type light source (lamp unit) including a high-pressure mercury discharge lamp and a reflecting mirror in combination has been known. The high-pressure mercury discharge lamp, in which a vapor pressure of mercury during lighting rises to a range of 20 MPa to 35 MPa so as to intensify continuous light emission in a visible region, is used.
Such a discharge lamp with a high pressure during the lighting has a possibility of generating a problem of breakage during the lighting due to a slight crack existing in a light-emitting tube glass portion or structural stress concentration, because of its high pressure. In order to prevent such a breakage problem, it is suggested to improve a structure of a metal foil that is sealed inside a sealing portion so as to suppress the generation of a crack around the metal foil. For example, Patent Document 1 discloses forming an edge portion of the metal foil to have a roundly curved shape that is convex toward its outside such that the stress is not concentrated.    Patent Document 1: JP 11 (1999)-111226 A